Over the past year 200 individuals in the Longitudinal Program have been included in the survey of immune function. On a cross sectional basis the preliminary findings are the "T" cell function as measured by proliferative and cytotoxicity assays starts to decrease in the fifth decade of life. Antibody production, a B cell assay, remains stable throughout life. A nonspecific host defense system, the granulocyte function drops off with age beginning at the 4th decade. Lymphocyte numbers in the peripheral blood start a steady age related decline starting at the third decade. In the coming year, we will begin to restudy individuals for the second time to determine the stability of the results obtained this year.